1. Field of the Invention
The present invention relates to a computed tomography scanner apparatus (to be simply referred to as a CT scanner hereinafter) and, more particularly, to a scan planning procedure or a reconstruction planning procedure of a CT scanner adopting a helical scanning scheme.
2. Description of the Related Art
Conventionally, when an object to be examined is scanned by an X-ray CT scanner, a so-called scan planning procedure or a reconstruction planning procedure is performed to suppress unnecessary X-ray radiation to the object and improve diagnostic efficiency by reconstructing only a tomographic image of a desired slice portion. In accordance with the scan planning procedure, it is determined before scanning which portion of the object is to be scanned to acquire projection data. In accordance with the reconstruction planning procedure, it is determined a tomographic image of which slice is to be reconstructed from the acquired projection data.
In the scan planning procedure, first, an X-ray tube is not rotated around the object but fixed immediately above or immediately beside the object. X-rays transmitted through the object are acquired while moving only the top plate of the bed in the direction of the body axis, and a scannogram of the object obtained by looking the object from the above or from its side is displayed. A scanning range is designated by using a line cursor on the scannogram. Then, the top plate is moved to cause the scanning position of the object to coincide with the start position of the designated scanning range. The top plate is then stopped and the X-ray tube is rotated around the object. Ordinary scanning is performed and projection data of the scanning position is acquired. After this, the top plate is moved at predetermined pitches. The top plate is stopped at every pitch shift and ordinary scanning is performed in the same manner. As a result, projection data acquired in units of scanning positions (top plate shift positions) are stored in an acquisition data memory.
When projection data of the entire scanning range are acquired, a slice to be reconstructed is designated again on the scannogram. Then, projection data at a scanning position corresponding to the designated slice is read out from the acquisition data memory, and a tomographic image of the designated slice is reconstructed based on the readout data.
An X-ray CT scanner of a helical scanning scheme has recently been developed in order to scan a large portion of an object at a high speed. In accordance with the helical scanning scheme, the top plate of the bed on which the object is placed is continuously moved in the direction of the body axis, and the X-ray tube is continuously rotated around the object, thereby scanning the object. In helical scanning, assuming that the object is stationary, the path of the relative movement of the X-ray tube seen from the side of the object becomes helical. In a CT scanner, in order to reconstruct a tomographic image of a certain slice, projection data of 360.degree. (in the case of half scanning, 180.degree.+fan angle) at a scanning position corresponding to the slice is required. In the case of helical scanning, however, since the top plate is also moved, only projection data of a single projection angle can be obtained at each scanning position. For this reason, projection data of other projection angles that cannot be obtained at a certain scanning position must be obtained by interpolation from two projection data obtained at the scanning positions on the two sides of the certain scanning position that have the same projection angle. The interpolation requires projection data for predetermined angles, for example, 360.degree.. As a result, projection data for angles are necessary to reconstruct the tomographic image in the helical scanning scheme.
On the scannogram, however, the range corresponding to the projection data for angles necessary to reconstruct the tomographic image cannot be easily known. Therefore, a scanning range cannot be correctly designated on the scannogram with an X-ray CT scanner of the helical scanning scheme. For this reason, excessive X-rays may be radiated onto the object to be examined and unnecessary projection data may be acquired. Though the positional relationship between the scannogram data and the projection data is known, it is necessary to consider that auxiliary data for interpolation is required in addition to main data for reconstruction in order to reconstruct the tomographic image with an X-ray CT scanner of the helical scanning scheme. In the conventional reconstruction planning procedure, the tomographic image cannot be reconstructed at the end portion of the scannogram, since the auxiliary data is not considered.